An image sensor is a device capturing and converting an impinging electromagnetic radiation such as a light flux into an electronic signal. In digital imaging, Active-Pixel Sensors (APS) are mostly used. APS are image sensors consisting of an integrated circuit containing an array of pixel sensors, and wherein each pixel contains a photodiode and an active amplifier. There are many types of Active-Pixel Sensors, including the Complementary Metal Oxide Semiconductor (CMOS) APS, used most commonly in cell phone cameras or web cameras for instance. Such image sensors have emerged as an alternative to Charge-Coupled Device (CCD) image sensors.
In an APS, the photodiode is sensitive to incident light. More precisely, the photodiode converts the incident light into charges which are accumulated during a given exposure time and then converted into an amplified voltage inside the pixel. This voltage is a continuous analog physical quantity which can be converted, thanks to an analog-to-digital converter, to a digital number representing the voltage amplitude. The terminals of the cathode of the photodiode, referred to as the node attached to the cathode, is often called the detector node. This node (20) is displayed in FIG. 1 illustrating an APS pixel. The voltage at this node is translated towards the pixel output via a transistor used as an amplifier. One important figure of merit characterizing pixels is their so-called fill factor. It refers to the portion of area sensitive to the light, in percentage, among the entire pixel area. FIG. 2 discloses a typical pixel area, divided in a photosensitive area 1 and a circuitry area 2.
One of the major disadvantages of standard pixels is their potential saturation appearing when too strong incident light and/or too long exposure occur. In range imaging system using Time of Flight technologies (ToF), for example a Time-of-flight camera system providing distance information by analysing the Time of Flight and the phase of a pulsed light signal emitted by a controlled light source and reflected back by objects from the scene, the saturation may occur when objects having standard reflective properties are closer from the distance range the imaging system is calibrated for. The object reflects at that time too much from the emitted light and causes at least some pixels of the sensor to respond at their maximum value. The saturation may also occur when objects demonstrates specular reflective properties in the wavelength domain the pixels have been designed to be sensitive to, such as when a mirror in a scene reflects the entire incident light it receives onto the sensor imaging the scene, or when objects reflect and concentrate the incident light onto a portion of the sensor, or when an external light source emitting a strong illumination in the same wavelength domain the ToF camera has been designed for is illuminating the sensor. When pixels are saturated, meaningful information about the scene is lost since the response provided is flattened at the maximum voltage value that can be provided; this leads to image artefacts or defects such as burned area, blooming effects in images. Moreover, certain applications, for instance the computation of depth information in ToF technology, use phase shift based computations from a plurality of captures to derive a distance measurement. If pixel saturation occurs during integration time, the voltage at the detector nodes reaches a saturation level which corrupts the corresponding capture. This more particularly makes relative voltage amplitudes determination in between the different phase impossible and as a consequence, the depth measurements and the corresponding depth map cannot be determined anymore as it is usually derived directly from these phase difference computations.
For the purpose of overcoming saturation issues, amongst other things, High Dynamic Range (HDR) or Wide Dynamic range (WDR) have been proposed in standard image sensors using several electronic circuits, for instance Schmitt trigger (as defined herein below) with addition of latches and/or memory point. Sensors have also been designed with technics such as well adjusting, multiple captures or spatially varying exposure. Moreover, extra logic circuitry has been added per CMOS APS, but this reduces the effective sensitive area of sensor and results in a very low fill factor that do not comply with efficient ToF imaging requirements. Another solution consists in using circuits with logarithmic pixels. Such pixel circuits generate a voltage level that is a logarithmic function of the amount of light striking a pixel. This is different from most CMOS or CCD type image sensors that use a linear type of pixels. Nevertheless, the use of logarithmic pixels complicates highly the post processing to compute required data, as depth information for instance, since it introduces well known compression issues and request also extra processing computations.